This application claims the benefit of Japanese Patent Applications No.2001-162682 filed May 30, 2001 and No.2001-232336 filed Jul. 31, 2001, in the Japanese Patent Office, the disclosures of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to halftone screening methods and storage media, and more particularly to a halftone screening method which uses halftone cells of a rectangular lattice, and a computer-readable storage medium which stores a computer program for causing a computer to process image data according to such a halftone screening method.
2. Description of the Related Art
FIGS. 1A and 1B are diagrams for explaining a conventional scattered dot dither type halftone cell. In FIGS. 1A and 1B, a non-regular hexagonal halftone cell is created by cutting a pair of confronting corners of a square halftone cell into a pair of confronting sloping sides. Since the pair of confronting corners of the square halftone cell made up of 9×9=81 pixels are cut into the pair of confronting sloping sides, the non-regular hexagonal halftone cell is made up of 81−9=72 pixels. FIG. 1A shows a non-regular hexagonal halftone cell C for one color, namely, cyan, and shows a case where 3×3=9 pixels at the central portion are colored to represent a dot C1 having a gradation level “9” with respect to a maximum number “72” of gradation levels.
When centers of halftone dots of each of the colors are arranged at the same pixel position of the halftone screen in a color printer, even a slight error in the positions of the halftone dots of each of the colors on the halftone screen results in a conspicuous change in color tone. FIG. 1B shows a case where a non-regular-hexagonal halftone cell M for magenta is shifted by 1 pixel to the right and 1 pixel to the top with respect to the non-regular hexagonal halftone cell C for cyan, and a non-regular hexagonal halftone cell Y for yellow is shifted by 2 pixels to the right and 1 pixel to the bottom with respect to the non-regular hexagonal halftone cell C for cyan. In this case, even if the 3×3=9 pixels at the central portion of the non-regular hexagonal halftone cell M is colored to represent a dot M1, the 3×3=9 pixels at the central portion of the non-regular hexagonal halftone cell Y is colored to represent a dot Y1, and the dots C1, M1 and Y1 are overlapped, the positions of the dots C1, M1 and Y1 do not match and the change in the color tone is conspicuous.